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The Best Inline Aquarium Heater Options For Freshwater & Saltwater Tanks

Finding the best inline aquarium heater is a great way of heating your water evenly while reducing equipment in your tank!

By Julie Millis
Last updated on

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best inline aquarium heater

If you have a larger tank and want to reduce the amount of equipment on display, an external inline heater can be a great choice. But how do you find the best inline aquarium heater for your fish tank?

We’ve assessed some of the best inline aquarium heaters available and chose the Hydor Inline External Heater as our overall pick.

But we also reviewed some other options that will suit if you’re on a budget, or if you need a wider range of power sizes too.

So let’s get started!

  1. Best Overall: Hydor Inline External Heater
  2. Best Budget Pick: DaToo Aquarium External Inline Heater
  3. Best Size Range: ISTA Inline External Aquarium Heater

Best Inline Aquarium Heater Picks Reviewed

We assessed the best inline aquarium heaters to bring you our top picks below. Each has been reviewed against criteria including hose size fittings, safety features, and suitable tank size.

1. Hydor Inline External Heater


  • Heater Type: External inline, adjustable
  • Dimensions: 12.5 (l) x 4.0 (d) x 4.0 (w) inches
  • Hose Sizes: ½ inch, ⅝ inch
  • Wattage: 200 Watt, 300 Watt
  • Key Features: Temperature range of 65 to 93°F, built-in thermostat, electronic temperature dial, LED heating indicator, hanging eyelet
  • Best For: Overall

Hydor’s inline aquarium heater is compatible with freshwater and saltwater tanks and is our top choice overall. If you use a sump or canister filter in your larger aquarium, this heater is a great option.

It comes in a 200 Watt version with ½ inch or ⅝ inch hose connectors, and a 300 Watt model with ⅝ inch connectors. So whether you have a 40 gallon tank, a 55 gallon tank, or even a 75 gallon aquarium you’ll find a size to fit.

The electronic control dial is easy to turn, and the 2-degree increments make it simple to set the temperature accurately. You can adjust it anywhere from 65 to 93°F, and because it’s accurate to just +/- 1°F it’s perfect if you have sensitive marine fish or corals.

Inside the Hydor, it uses a clever PTC (Positive Thermal Coefficient) polymer. PTC is ‘self-limiting’ meaning once it reaches its maximum temperature it becomes non-conductive and won’t overheat. So great peace of mind to know you won’t endanger your fish.

There’s an LED heating indicator so you can keep an eye on when it’s actively heating and quickly identify any faults if it doesn’t come on when the temperature is too low. 

And there’s a handy hanging eyelet that helps you to hang the unit vertically to prevent any bubbles or dry areas inside the heater which can cause cracks.

While some users have reported leaks during initial setup, this has usually been due to needing to get a good seal when installing. But the majority of users praise this heater for its ability to heat their water evenly while leaving their tank clutter-free.

2. DaToo Aquarium External Inline Heater


  • Heater Type: External inline, adjustable
  • Dimensions: 12.0 (l) x 2.7 (d) x 1.8 (w) inches
  • Hose Sizes: ½ inch, ⅝ inch, 20mm (0.78 inch)
  • Wattage: 300 Watt
  • Key Features: Temperature range of 68 to 95°F, built-in thermostat with 2 temperature sensors, glass-ceramic shatterproof heating tube, LED display and heating indicator
  • Best For: Budget Pick

The DaToo inline aquarium heater is another good choice if you own a canister filter and especially if you are looking for a budget option. It offers 300 Watts of power and so is perfect if you own around a 75 gallon aquarium.

DaToo provides 3 different connection adaptors, ½ inch, ⅝ inch, and 20mm (0.78 inches), so it will fit a range of canister outflow hoses. And if you change your canister size in the future you can simply swap the adaptor.

We found the digital temperature display and single temperature adjustment button easy to use, and with a range of 68 to 95°F you can set it to suit most tropical fish. 

It has 2 temperature sensors feeding the built-in thermostat, which shuts down the heater once it’s 0.2°F above the set temperature. So you can be confident it won’t overheat your water to dangerous levels or cause harm to your fish.

Some owners report the set temperature can sometimes be out by +/- 1 or 2 degrees, so do ensure you keep a separate in-tank thermometer to check it. But once it is set, many give the DaToo good feedback for its ease of use and ability to maintain a stable temperature.

3. ISTA Inline External Aquarium Heater


  • Heater Type: External inline, adjustable
  • Dimensions: 8.75 (l) x 2.2 (w) inches
  • Hose Sizes: ½ inch, ⅝ inch
  • Wattage: 150 Watt, 300 Watt, 500 Watt
  • Key Features: Temperature range of 68 to 95°F (20 to 35°C), built-in thermostat with 2 temperature sensors, auto shut-off if heater runs dry or temperature is 3°C over set temperature or over 36°C, LED heating indicator, LED overheat warning light
  • Best For: Size Range

The ISTA Inline Aquarium Heater has the widest size range of those featured, with 150 Watt, 300 Watt, and 500 Watt models. So whether you have a 40 gallon fish tank or a 150 gallon tank, there’s a size to suit.

Each Wattage comes in a version with either a ½-inch or ⅝-inch hose connector, so make sure you choose the correct one for your canister or sump outflow hose. And each model is also compatible with freshwater or saltwater tanks.

You can adjust the temperature range from 68 to 95°F (20 to 35°C) using the simple touch button control. But be aware, whilst this heater is provided for the American market with US plugs, it only shows the temperature in Celsius so can take some getting used to.

Like the DaToo, the ISTA has 2 temperature sensors helping it to maintain accuracy to +/- 1°C, so it’s great for sensitive fish or marine animals in a saltwater tank. It also has an LED indicator to show when it’s heating, and a warning light so you can see if it’s overheating.

There are plenty of safety features on the ISTA. It has an auto shut-off if the temperature runs 3°C above what you’ve set, or if it goes over 36°C. It will also shut off if it runs dry for a short period, so if your canister or pump stops working you won’t damage your heater.

Some owners report the hose connectors can leak on the ISTA, so you may even need to buy hose clamps to ensure a tight seal. But overall many give positive feedback for the accuracy and ease of use.

Guide To Finding The Best Inline Aquarium Heater

Depending on the type and size of your tank, an inline aquarium heater can be a fantastic option. Let’s look at some of the key considerations to think about before you choose yours.

large freshwater planted tank

How Do Inline Aquarium Heaters Work?

An inline aquarium heater is an external heater you install on the outflow line from your canister filter or sump. As the water is pumped back to your tank it passes through the heater where it’s heated to temperature before being returned to your aquarium.

Inline aquarium heaters contain a heating element, a thermostat, and either a digital display or analog dial for setting the temperature. Once your water is at the correct temperature, the thermostat will detect it and switch off the heater until the temperature falls again.

Are Inline Heaters Better Than Submersible Heaters?

Inline aquarium heaters are one of the most efficient types if you have a larger fish tank. Because they heat the water as it’s pumped back to your tank, they are very effective at providing even heating and a consistent temperature throughout.

Submersible heaters placed in your tank can also be highly efficient as long as they are positioned correctly near your filter outflow to evenly distribute the heated water. If you have a freshwater 30 gallon tank, for example, a single submersible heater can be a great choice.

Overall, submersible heaters are more appropriate for smaller fish tanks rather than inline heaters. But either type can be very effective for larger aquariums. It really depends on your setup and individual preferences.

What Are The Pros And Cons Of Inline Aquarium Heaters?

While both submersible and inline aquarium heaters can be very efficient, inline aquarium heaters do have their individual pros and cons to consider.

Pros Of Inline Aquarium Heaters

Here are some of the key benefits of choosing an inline aquarium heater for your tank.

  • Less In-Tank Equipment: An external inline heater means you can hide it in your aquarium cabinet, rather than having it on display in your tank. This is perfect for aesthetics as well as maximizing swimming space for your fish.
  • Accurate And Even Heating: As inline aquarium heaters heat your water as it’s pumped back to your tank it means it’s evenly heated throughout. The even heating and cycling also make thermostat readings very accurate as the water flows through.
  • No Need To Unplug: If you own a submersible heater you need to switch it off before performing a water change to prevent it running dry and potentially shattering. Inline aquarium heaters can be left running though, making water changes easier and maintaining the temperature while you carry out your tank maintenance.
  • No Heating Element Near Aquatic Life: Often a submersible heater will need a bumper guard or casing that protects your fish from coming into contact with the heating element. With an inline aquarium heater outside your tank, there’s no danger of your fish or plants coming into contact with it or any injuries happening.

Cons Of Inline Aquarium Heaters

But inline aquarium heaters do also have some potential disadvantages to be aware of.

  • Risk Of Leaks: Unfortunately inline aquarium heaters can sometimes be prone to leaking, often as a result of incorrect installation. You can minimize this risk by ensuring you buy the correct version for your canister hoses, often ½ or ⅝ inch sizes, and checking the fitting is secure during setup.
  • No Heating Without Flow: As your inline aquarium heater heats the return water from your filter or sump if your pump fails or the flow stops your tank won’t be heated. Inline heaters can also be at risk of overheating if this occurs. 
  • Higher Price: Inline aquarium heaters are often around twice the price when compared to submersible heaters. So whereas a 200 Watt submersible heater may cost around $30, a 200 Watt immersible aquarium heater could be closer to $60.
  • Harder To Install: As mentioned above, inline aquariums need to be carefully installed to minimize the risk of leaks. They can also slightly restrict the flow of the return water from your canister so adjusting your pump correctly during setup is key.

How Do You Install An Inline Aquarium Heater?

It’s important to carefully set up your inline aquarium heater to avoid any leaks or issues. And while you should follow the instructions for your individual heater, here are some key tips to make installation easier.

  • Install On The Outflow: Always install your inline aquarium heater on the outflow return from your canister filter or sump. This ensures you’ll be heating clean, filtered water so your heater won’t get clogged with debris. It also means heated water will flow straight back to your tank. Whereas installing it on the inflow line would mean the water passes through your filter, so losing some of its temperature.
  • Soak Your Hose When Fitting: Often attaching your outflow hose to the heater connection can be a very tight fit. To help with this, soak the end of your hose in hot water first. This makes it easier to slide onto the connector and it will also seal tightly once it cools for a better fit. Always check the connector grip or hose clamp is tightened too.
  • Always Install Vertically: It’s critical to make sure your inline aquarium heater hangs vertically when set up. Leaving it in a horizontal position can cause bubbles to occur which can leave dry areas in the heater’s glass casing, and eventually cause cracks.
  • Use A Heater Controller: Some aquarists recommend you use an aquarium heater controller as a failsafe in case your inline aquarium heater doesn’t switch off when it should. Whilst it’s not essential and could happen with any type of heater, it’s a good precautionary measure.

How To Choose The Best Inline Aquarium Heater For You

Before you choose the best inline aquarium heater for your tank, let’s look at the key things to look out for.

discus in large aquarium

Size And Type Of Aquarium

An inline aquarium heater is usually suitable if you have a 40 gallon fish tank or larger, although occasionally you can find them for smaller aquariums. The majority are compatible with both freshwater and saltwater fish tanks.

In fact, if you have a reef tank with marine fish or corals, an inline heater can be a great choice as it will provide even heating of your water. This is important as corals are particularly sensitive to fluctuations in temperature [1].

If you keep an aggressive tank, for example, cichlids like Duboisi Cichlid, an external inline aquarium heater can also be preferable. Having less equipment in your aquarium means fewer potential knocks and less damage to your heater or injuries to your fish.

Wattage Vs. Tank Volume

Inline aquarium heaters tend to heat your water very efficiently and can run slightly hotter than other types. However, you should still follow the general rule that around 2.5 to 5 Watts are needed to heat 1 gallon of water [2].

So a 40 gallon tank would need an inline aquarium heater of around 150 to 200 Watts. A 300 Watt inline heater is suitable for around a 75 gallon aquarium, and a 500 Watt is suitable for around a 150 gallon tank.

This is a general guide though as your climate and room temperature will also make a difference to the power needed. So if in doubt, refer to the manufacturer’s recommendations for the heater you are considering.

Temperature Control And Adjustability

Many of the best inline aquarium heaters come with a digital display to show you the current and set temperature, although some do come with an analog dial. As long as the dial is easy to read, in say 1 or 2-degree increments, then accuracy shouldn’t be an issue.

Inline aquarium heaters tend to have a good range of adjustability, often in the range of around 65 to 95°F. Check to see that your chosen heater either has easy-to-use touch-button controls if it’s digital, or if it’s analog, that the dial is easy to rotate and set.

Safety Features

Inline aquarium heaters are generally one of the safest types as their construction and outer casing mean they can’t shatter if they overheat. However, there are some additional safety features we recommend you look for.

Check to see whether your chosen heater has an auto switch-off that will turn it off if it senses no water is running through. This is an important failsafe in case your sump or filter pump stops working for any reason.

The best inline aquarium heaters also tend to have an indicator light to show when they are actively heating. By using this along with an in-tank thermometer, you can easily tell if there is a fault if your heater doesn’t come on when the temperature drops.

Our Verdict

If you own a large tank an inline aquarium heater can be a great way of reducing your in-tank equipment so you can focus on your fish.

We found the Hydor Inline External Heater to be the best inline aquarium heater for its build quality, accuracy, and even heating.

So whether you have a freshwater or saltwater aquarium, you’ll be able to find the best inline aquarium heater for your tank from our picks above!


Here are the answers to some of the most frequently asked questions when it comes to choosing the best inline aquarium heater.

What Is The Most Efficient Aquarium Heater?

Submersible heaters can be one of the most efficient types. However, inline aquarium heaters are also excellent for heating a large aquarium evenly and consistently.

Is An Inline Heater Easy To Install?

Inline aquarium heaters are often more tricky to install than submersible heaters. The best tip is to install vertically to avoid air bubbles and make sure connections are tight to avoid leaks.

What Size Inline Heater Should I Use For My Tank?

The general rule is to use 2.5 to 5 Watts per 1 gallon of water. But this is a guide and can vary, so do refer to the recommendations from the manufacturer too.

How Do I Know My Inline Heater Is Working Properly?

Inline aquarium heaters often have an indicator light to show they are heating. But as with any heater, it’s always best to keep a separate thermometer in your tank to monitor the temperature.

Photo of author
Julie Millis
Julie has been involved in aquatics for over 15 years. She is passionate about freshwater and saltwater tanks. Julie loves helping with all your fish-keeping questions!

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